In the past, there have been many ways of attempting to prevent unauthorized duplication of important or sensitive documents. The development of xerography, and particularly color xerography, has provided the unscrupulous with the means for unauthorized duplication of original documents for the purpose of passing them off, with or without alteration, as an original document of the same kind. As a result of color xerography, the mere printing of an important document with a colored background is insufficient for preventing unauthorized duplication.
In fact, the problem is widespread and well known to the issuers of such original documentation which has led to considerable attention being given to ways to prevent the duplication of such documents by color xerography. While in no sense limited in practice, the documents will be understood to be of the type which include negotiable instruments, title instruments, identification instruments, and other similar security instruments or documents.
Because of these problems, there has been much attention given to the prevention of effective duplication of such documents by color xerography. It has become known, for instance, that xerographic copiers have a screen value, or dot frequency, threshold above which the copier is unable to distinguish the individual elements of the dot pattern of half-tone printing and that, as to color xerography, there are also spectral ranges of color in which the reproductive capability of the copier is relatively impaired. As a result of this knowledge of color xerography, the focus has been on using these facts to render unauthorized xerographic duplication difficult.
Typically speaking, the primary focus has been upon causing invalidating indicia of tampering to be essentially latent to the naked eye looking at the original document without the aid of magnification but to appear boldly in the xerographic copy.
In most earlier proposals, the invalidating indicia are printed in one dot frequency or screen value and the background is printed in another with the indicia camouflaged in one of a couple of different ways. In particular, this is achieved either with an intermediate third-dot frequency immediately surrounding the invalidating indicia or with a covering overlay of extraneous pattern intended to confuse the eye sufficiently to render the warning indicia indiscernible to ordinary observation. More recently, the invalidating indicia and the background have been printed in different screen values, one above and one below the dot frequency threshold of a xerographic copier, with the invalidating indicia as a compact all over pattern serving as its own camouflage.
While all of these techniques have their merits, there are certain important drawbacks that are inherent in each of them. These include the fact that there are continuously new improvements in the field of color xerography whereby exact reproductions of the original documents are or may become possible, i.e., the invalidating indicia may not appear in the xerographic copy. In order to keep pace, it is important to develop an improved security document that is independent of color xerography characteristics.
The present invention is directed to overcoming one or more of the foregoing problems and achieving one or more of the resulting objects.